This invention relates to highly biodegradable aminopolycarboxylates which are useful as chelating agents, a process for producing the same and use thereof.
As chelating agents for capturing metals contained in, for example, waste water, it has been a practice to employ electrolytic polymers such as polyacrylic acid and polymaleic acid, aminocarboxylates such as ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid, polyphosphates such as sodium tripolyphosphate. However, these conventional chelating agents are each poor in biodegradability. Therefore, it has been feared in recent years that they might exert undesirable effects on the environment.
Accordingly, there have been proposed various biodegradable chelating agents. JP-A-5-72695 discloses N,Nxe2x80x2-alkylenediaminedisuccinic acids, JP-A-8-165271 discloses 2-hydroxy-1,3-propanediaminepolycarboxylic acids, JP-A-9-87675 discloses unsaturated aminocarboxylic acids, JP-A-9-124567 discloses 2,2xe2x80x2-dimethyliminodiacetic acid derivatives, JP-A-8-188549 discloses glycerol derivatives, JP-A-8-92197 discloses 2-sulfoethylaminocarboxylic acids, and JP-A-7-89913 discloses aspartic acid derivatives (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d).
Moreover, JP-A-6-67370 and JP-A-7-64260 disclose ethylenediamine-N,Nxe2x80x2-dimalonic acid and its iron (III) complex salt as components of solutions for processing silver halide photographic materials. It is also disclosed in these references that these compounds have biodegradability. It is stated in these references that ethylenediamine-N,Nxe2x80x2-dimalonic acid was synthesized in accordance with Misao M. et al., Bull. Chem. Soc. Japan., vol. 46, pp. 844 to 847 (1973) (hereinafter referred to as the xe2x80x9cdocument 1xe2x80x9d) or USSR Patent No. 316685. A production process starting with a malonic acid salt is disclosed in the document 1 or USSR Patent No. 316685. No process but the one described above has been known hitherto as a process for producing ethylenediamine-N,Nxe2x80x2-dimalonic acid.
However these biodegradable chelating agents proposed so far are unsatisfactory in chelatability and chemical stability. To industrially use these chelating agents, it is therefore required to further improve the qualities thereof.
In case where ethylenediamine-N,Nxe2x80x2-dimalonic acid is produced with the use of a malonic acid salt as a starting material in accordance with the process described in the document 1 or USSR Patent No. 316685, the resultant compound is liable to degrade and ethylenediamine-N,Nxe2x80x2-diacetic acid is formed as a by-product by decarboxylation, thereby lowering the yield. Thus, this process is unsatisfactory from an industrial viewpoint.
It is stated in the document 1 that although disodium ethylenediamine-N,Nxe2x80x2-dimalonate was produced, the product has only a poor chelatability due to the steric hindrance of the structure of this compound. It is also stated therein that a tetraalkali metal salt of ethylenediamine-N,Nxe2x80x2-dimalonic acid is liable to be hydrolyzed. The document 1 presents neither any example of the isolation of the tetrasodium salt nor any example of the production thereof.
JP-A-7-64260 discloses use as a bleaching agent and neither the tetrasodium salt nor use thereof as a chelating agent is described therein. In this reference, the iron (III) complex salt of ethylenediamine-N,Nxe2x80x2-dimalonic acid was synthesized in accordance with the document 1 and ethylenediamine-N,Nxe2x80x2-dimalonic acid or disodium salt thereof was used as an intermediate in producing the iron (III) complex salt of ethylenediamine-N,Nxe2x80x2-dimalonic acid.
The invention, which has been completed by taking the above-described problems into consideration, aims at providing chelating compounds, which are superior in chelatability and chemical stability to the chelating agents proposed hitherto and have high biodegradability, a process for producing the same and use thereof.
By considering the problems as described above, the inventors have paid attention particularly to the chelatability and biodegradability of aminocarboxylic acid chelating agents and conducted intensive studies. As a result, they have found out:
1) an aminopolycarboxylate represented by the following formula (1): 
wherein M1, M2 , M3 and M4 independently represent each an alkali metal ion has a high chelatability and an excellent biodegradability and the chemical stability can be extremely elevated by forming alkali metal salts of all of the four constituting carboxylic acids;
2) in producing this aminopolycarboxylate, the aminopolycarboxylate represented by the formula (1) can be obtained in a high purity and at a high yield by reacting ethylenediamine with a malonic acid ester derivative in a liquid containing alkali metal(s) and recrystallizing the alkali metal aminopolycarboxylate thus formed from a solvent mixture of water with a poor solvent compatible with water, or crystallizing by adding a poor solvent compatible with water to an aqueous solution of the alkali metal aminopolycarboxylate thus formed;
3) the highly pure aminopolycarboxylate obtained by this process is highly stable and undergoes substantially no degradation even under heating at 50xc2x0 C.; and
4) this aminopolycarboxylate is excellent in the ability to capture metals contained in, for example, waste water. The invention has been completed on the basis of these findings.
Accordingly, the invention relates to aminopolycarboxylates represented by the formula (1) as described above, a process for producing the aminopolycarboxylates by reacting ethylenediamine with a malonic acid ester derivative, and use the aminopolycarboxylates such as chelating agents containing the same as the active component.